1. Field of the Invention
The present invention relates to a printing device and, in particular, to a dot printing device for marking dots, in the form of characters, on a printing paper sheet by selectively using printing needles.
2. Description of the Related Art
Conventionally, there is known a dot printing device which is equipped with a printing head having a plurality of printing needles arranged in a vertical direction as a needle line and performs a printing operation by electromagnetically driving the needle line while moving the printing head in a horizontal direction. The dot printing device has been used as an output terminal of, for example, a computer and a measuring instrument. Various functions and performances have been required on dot printers in accordance with their manner of use and the kind of apparatus and devices to be connected. A further improvement is expected regarding the printing speed, printing quality, and so on.
A high-speed printing operation can be implemented by increasing the travel speed of, for example, the printing head. In this case, the driving cycle of the printing needle is made short according to an increase in the travel speed of the printing head. In order to determine the driving cycle, however, it is necessary to take the response characteristic of the printing needle and the heat-radiation capacity of the printing head into an account. If these factors are disregarded, there is a possibility that the printing needles will fail to mark dots and that the printing head will be overheated. When the driving cycle of the printing needle is shortened to a period limited by the response characteristic of the printing needle and the heat-radiation capacity of the printing head, it is difficult to further increase the travel speed of the printing head.
A technique is known which allows the travel speed of a printing head to be increased even if the driving cycle of the printing needle is set to that period. In this technique, a plurality of printing needles are arranged as two needle lines in a vertical direction perpendicular to the direction of movement of the printing head such that each printing needle of one needle line is positioned at the same height as a corresponding needle of the other needle line in the vertical direction. When the printing head is used to print characters by one line, the number of drives of each needle line is reduced to one half its number in the case where a printing head having a single needle line is used. It is thus possible to double the travel speed of the printing head without changing the drive cycle of the needle line.
The printing quality depends upon, for example, the density of dots to be marked with the printing needle. The higher the density, the clearer the characters. In order to reduce the pitch of dots in a vertical direction, a printing head has been suggested which has a plurality of printing needles arranged as two needle lines in the vertical direction with each printing needle of one needle line positioned higher in the vertical direction than a corresponding one of the other needle line. In the printing operation using the printing head, the drive timing of the two needle lines is so controlled that the dots marked by the printing needles of one needle line can be located in the spaces between the dots marked by the printing needles of the other needle line. The pitch of the dots in a horizontal direction can be reduced by lowering, for example, the travel speed of the printing head without changing the driving cycle of the needle line.
It is desirable that an inexpensive dot printer can perform both of a high-speed printing and a high-quality printing. When the techniques mentioned above are used to improve the printing speed and printing quality, the printing head and control mechanism may become complex and bulkier. The whole size of, for example, the printing head is principally determined by the size of a heat sink for dissipating heat which is involved when the printing needle is driven. Even if the number of printing needles is slightly increased, the heat sink becomes considerably bulkier. It is, therefore, difficult to implement the aforementioned dot printer at low costs. In view of the aforementioned situation, many users have to select either one of a high-speed printing type and high-quality printing type from among dot printers in accordance with the necessity.